This invention relates to a method for reinforcing a conical shaped object and object formed therewith and, more particulaly, to a method for reinforcing a conical shaped object with fiber elements and to a conical preform formed from fiber elements, wherein the fiber volume fraction along the axis and circumference of the conical surface remains invariant.
In certain applications, like when an object is expected to be exposed to a relatively harsh environment, typically a composite material is used to form the object or to be applied to surfaces of the object for protection against the environment and/or for reinforcing the object. It is desirable that the composite material have a substantially constant fiber element reforcement fraction over the surface of the object so that significant composite property disparities between areas of the surface are avoided, thereby permitting accurate predictions of composite material response to the environment. It has been especially difficult to obtain a constant fiber reinforcement fraction along the axis of a conical or other axially increasing diameter shell structure.
Prior three-dimensional fiber reinforcement patterns have drawbacks when configured to form or conform to a conical surface. These include: failing to maintain constant radial and in-plane (i.e. conformed to the conical surface) fiber reinforcement fractions along the length of the conical surface while maintaining continuous paths for winding the in-plane fibers through a radially disposed fiber array; or failing to provide continuous paths for winding the in-plane portion of the fiber reinforcement material while maintaining a lesser variation of constant radial and in-plane fiber volume fractions along the length of the conical surface. For the former case, which is typical of three-directional reinforcement designs, significant variations in structural properties occur along the length of the conical surface since the radial and in-plane fiber reinforcement fractions vary with axial position along the conical surface. In the latter case, discontinuities in the in-plane fiber reinforcement paths results in structural deficiencies and make fabrication of a fiber reinforcement preform impractical.
U.S. Pat. No. 4,519,290--Inman et al. discloses a three-dimensional four directional (4D) braided preform fabrication for making annular or conical sections to be used in producing articles. The 4D fiber architecture includes a plurality of rods of carbon fibers uniformly distributed over the surface and inserted into a conical mandrel perpendicular to the conical centerline as shown in FIG. 2 of the patent. Oblique carbon or graphite fibers are then passed alternately over and under similar longitudinal fibers around the radially extending rods to provide a triaxial braided pattern having a repeating unit cell that is illustrated in FIG. 6 of the patent. However, the 4D fiber architecture described in U.S. Pat. No. 4,519,290 does not achieve invariance of fiber volume fraction along the conical surface.
A 4D triangular fiber arrangement is described in a DTIC report ADB049350 entitled "Boron Nitride--Boron Nitride Composite Material" by Potter and Place. FIG. 4 of the Potter and Place report illustrates a cylindrical configuration having three triangularly related fibers disposed in a plane perpendicular to the axis of the cylinder and one fiber disposed in a plane parallel to the axis of the cylinder. This fiber arrangement would not generate a constant fiber volume fraction of radial fibers over a conical shell, nor would a constant fiber volume fraction be obtained in the conical surface direction of the shell without addition of new fiber ends.
U.S. Pat. No. 4,570,166--Kuhn et al., describes conformal mapping of a planar sector of a circle, having a grid pattern of isosceles triangles inscribed therein, onto the surface of a cone corresponding to the sector in the context of an RF transparent conically shaped antenna shield structure. The vertices of the triangles are used to situate RF components in the antenna shield structure.
Accordingly, it is an object of the present invention to provide a method for forming a three dimensional fibrous element preform having a conical surface, wherein the preform includes an invariant fiber volume fraction along with the axis and circumference of the conical surface.
Another object is to provide a method for reinforcing an object having a conical surface, wherein a single fiber element may be used to form the in-plane fraction while obtaining invariant fiber fraction along the axis and circumference of the conical surface.